.github: prevent duplicate builds when ci:main label is added to PRs

Add concurrency control to trigger workflow to cancel in-progress builds
when new events (like adding ci:main label) occur on the same PR.  This
prevents resource waste from duplicate builds and handles the common
workflow where developers add the ci:main label after PR creation.

Fixes #1154

.github/workflows/trigger.yml

@@ -9,6 +9,10 @@ on:
       - ci-work
   workflow_dispatch:
 
+concurrency:
+  group: ci-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build-x86_64:
     if: startsWith(github.repository, 'kernelkit/')